An outer membrane surface protein has been isolated from a number of strains of mycobacterial including M. tb which agglutinates RBCs. This protein can be purified using heparin-Sepharose chromatography. Monoclonal antibodies to this protein stain the surface of M. tb suggesting it is a major surface antigen. Preliminary studies indicate that it may function as a bacterial adhesin and ongoing studies will investigate the nature of bacterial host interactions including the identification of eukaryotic receptors for this factor.